The present invention relates to a piezoelectric/electrostrictive material made of a porcelain obtained by firing, for example, to a piezoelectric/electrostrictive material used as an actuator or a sensor both assembled as an electromechanical transducer for positioning in precision machine tool or length control of optical path in optical instrument or in valve for flow rate control, etc. More particularly, the present invention relates to a piezoelectric/electrostrictive material suitably used in a very small sensor or a highly integrated very small actuator both used in an element for measurement of liquid property or very small weight.
As piezoelectric/electrostrictive materials, there have been known Pb(Zr,Ti)O3 (hereinafter referred to as PZT), BaTiO3, etc. They are in use in actuators, filters, various sensors, etc. PZT type piezoelectric/electrostrictive materials have been used mainly because they are superior in overall piezoelectric properties.
Pb contained in PZT, etc. is stabilized and essentially generates no problem caused by decomposition or the like. However, there are cases that a Pb-free material is required depending upon its application. Further, since Pb-containing porcelains such as PZT, PLZT [(Pb,La)(Zr,Ti)O3] and the like give rise to vaporization of small amount of Pb in high-temperature firing, they have had, particularly when used in applications requiring a thin or thick film, a problem that they hardly show stable properties owing to the compositional change during firing.
Meanwhile, BaTiO3 contains no Pb and offers a promising material for such a need. BaTiO3 viewed as a piezoelectric/electrostrictive material, however, is inferior in piezoelectric/electrostrictive properties to a PZT type material, and has seldom been used as an actuator or as a sensor.
The present invention has been made in view of the above-mentioned problems of the prior art and aims at providing a BaTiO3-based piezoelectric/electrostrictive material which is superior in piezoelectric/electrostrictive properties to conventional products and which can be suitably used in an actuator or a sensor, and a process for producing such a piezoelectric/electrostrictive material.
A piezoelectric/electrostrictive material, when used in an actuator, is required to show a large displacement to a voltage applied. A study by the present inventor, made on the piezoelectric/electrostrictive properties of BaTiO3-based porcelain indicated that by controlling the fine structure of BaTiO3-based porcelain, particularly the distribution of the crystal grain constituting the BaTiO3-based porcelain, a piezoelectric/electrostrictive material showing a large displacement can be obtained. This finding has led to the completion of the present invention.
According to the present invention, there is provided a piezoelectric/electrostrictive material made of a BaTiO3-based porcelain composed mainly of BaTiO3 and containing CuO and Nb2O5, characterized in that 85% or more of the crystal grains constituting the porcelain are grains having particle diameters of 10 xcexcm or less and the maximum particle diameter of the grains is in a range of 5 to 25 xcexcm.
In the piezoelectric/electrostrictive material of the present invention, at least Dart of the Ba may be substituted with Sr. Also in the present invention, the Ba/Ti ratio or the (Ba+Sr)/Ti ratio is preferably in a range of 1.001 to 1.01 because such a ratio can easily prevent the growth of abnormal grains occurring during the firing for porcelain formation and can easily control the particle diameters of the crystal grains constituting the porcelain.
According to the present invention, there is also provided a process for producing a piezoelectric/electrostrictive material made of a BaTiO3-based porcelain composed mainly of BaTiO3 and containing CuO and Nb2O5, characterized by weighing individual raw materials so as to give a predetermined composition, mixing and grinding them, calcinating the resulting mixed powder in the air at 850 to 950xc2x0 C., then grinding the resulting calcinated material until the ground material comes to have a specific surface area of 7 m2/g or less, and molding and firing the ground material.
The piezoelectric/electrostrictive material according to the present invention is described in more detail below. The piezoelectric/electrostrictive material according to the present invention is made of a BaTiO3-based porcelain composed mainly of BaTiO3 and containing CuO, Nb2O5, etc.
A specific composition of the porcelain of the present invention may be such wherein BaTiO3 is the main component and part of the Ba, for example, 0.1 to 10 mole % may be substituted with Sr. Also, the porcelain of the present invention may inevitably contain Zr, Si, Al, etc. in an amount of 0.5% by weight or less based on the total weight. Further in the BaTiO3-based porcelain of the present invention, the A/B ratio, which is a (Ba+Sr)/Ti, is preferably larger than 1, more preferably in a range of 1.001 to 10.1. Also, to the present porcelain are preferably added Nb2O, and CuO each in an amount of 0.05 to 0.5% by weight, more preferably each in an amount of 0.1 to 0.3% by weight based on the porcelain components excluding these components. Further, to the porcelain of the present invention may be added rare earth metals and/or transition metals other than the above components, in a total amount of 0.5% by weight or less in terms of their metal oxides. Incidentally, the forms of the components added are ordinarily oxides, carbonates or sulfates thereof.
The individual crystal grains constituting the porcelain of the present invention have crystal lattices of perovskite structure. The porcelain of the present invention is characterized in that the particle diameter distribution of the crystal grains constituting the porcelain is controlled as predetermined; specifically, 85% or more of the crystal grains are constituted by grains having particle diameters of 10 xcexcm or less and the maximum particle diameter of the grains is in a range of 5 to 25 xcexcm. In a preferred particle diameter distribution of the crystal grains, 90% to less than 100% of the crystal grains have particle diameters of 10 xcexcm or less and the maximum particle diameter of the grains is in a range of 10 to 25 xcexcm.
The action mechanism for why a porcelain having the above particle diameter distribution shows superior piezoelectric/electrostrictive properties, is not clear. However, from the results shown in Examples described later, it is clear that a porcelain constituted by crystal grains having a particle diameter distribution in the above mentioned range is superior in piezoelectric/electrostrictive properties to a porcelain having a particle diameter distribution in other range.
Next, description is made on the process for producing a piezoelectric/electrostrictive material according to the present invention.
First, raw materials (oxides, hydroxides and carbonates of metal elements) are weighed as so as to give a compositional range of the present invention and are mixed using a mixer such as ball mill or the like. In this mixing, it is preferred to allow the primary particles of each raw material after mixing to have particle diameters of 1 xcexcm or less, in order to allow the porcelain obtained to have a particle diameter distribution specified in the present invention.
Then, the resulting mixed powder is calcinated in the air at 850 to 950xc2x0 C. to obtain a calcinated material. An appropriate calcination temperature is 850 to 950xc2x0 C. With a calcination temperature above 950xc2x0 C., the resulting sintered material is nonuniform and, with a calcination temperature below 850xc2x0 C., an unreacted phase remains in the resulting sintered material, making it impossible to obtain a dense porcelain.
Next, the calcinated material obtained is ground using a grinder such as ball mill or the like until the ground material comes to have a specific surface area of preferably 7 m2/g or less, more preferably 5 m2/g or less. The ground material is molded by a monoaxial press and then by a hydrostatic press to obtain a molded material of desired shape. The molded material is fired at 1,100 to 1,250xc2x0 C. to obtain a sintered material. The most appropriate firing temperature is 1,150 to 1,200xc2x0 C.
In the above-mentioned production process, it is important to control the Ba/Ti ratio of BaTiO3 [the (Ba+Sr)/Ti ratio when part of the Ba has been substituted with Sr] depending upon the kinds and amounts of the components (e.g. CuO and Nb2O5) added to the main component BaTiO3. The Ba/Ti ratio [or the (Ba+Sr)/Ti ratio] is appropriately controlled so that an intended crystal grain diameter distribution can be obtained depending upon the amounts and forms (e.g. salt or metal) of the components added, the firing temperature, etc.
The sintered material (porcelain) obtained by firing is subjected to a polarization treatment and then allowed to stand for 24 hours or more, whereby the resulting material has a high strain property. The piezoelectric/electrostrictive material according to the present invention is superior in displacement property; therefore, it is useful as a general electromechanical transducer and is suitably used in an actuator, a sensor, etc.